Non-monotonic response of macrophages to mechanical stretch impacts skin wound healing

The skin is subjected to constant mechanical stress in both healthy and wounded states. Macrophages play crucial roles in skin homeostasis and in all stages of the wound healing process. However, the effects of static mechanical stretch (MS) on macrophages and the subsequent consequences on skin cells remain largely unclear. We applied static MS at amplitudes of 7%, 15%, and 21% to macrophages derived from THP-1 using a customized cell-stretching device, thoroughly investigating its impacts on viability, polarization, secretome, and underlying signaling pathways. Recognizing the substantial influence of the macrophage secretome on neighboring cells, we collected conditioned medium from macrophages exposed to MS (MS-CM) and evaluated its effects on keratinocytes, fibroblasts, and endotheliocytes. Macrophages exhibited a non-monotonic biological response to MS across the range of 7-21%, resulting in similar non-monotonic effects of MS-CM on the behaviors skin cell behaviors. The most significant effects were observed when macrophages were subjected to 15% MS. The 15% MS promoted macrophage viability and polarization toward the M2 phenotype, leading to increased release of anti-inflammatory cytokines and growth factors, as well as activation of the mechanotransduction pathways Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). Consistent with these findings, 15% MS-CM enhanced the migration of keratinocytes, endotheliocytes, and fibroblasts, and promoted in vitro tube formation and fibroblast activation. In contrast, both 7% and 21% MS showed a similar tendency but with less pronounced or insignificant effects. Additionally, in a full-thickness wound model, the application of concentrated 15% MS-CM demonstrated additional beneficial effects on wound healing by enhancing angiogenesis and dermal reconstitution. Our observation of the non-monotonic macrophage response to MS provides a foundation for elucidating how macrophages may translate mechanical cues into paracrine signals that influence skin function and wound healing dynamics.